abc's jim avila reports on how this tragedy unfolded. >> reporter: two hours after the winners crossed, 4:09 into the race, the amateur runners still filling boston's boylston street, two rapid-fire explosions at the finish line. >> something just blew up! [ explosion ] >> run! go! >> reporter: turning the cherished boston marathon into what one hospital official called a war zone. >> i crossed the 26-mile marker and i saw the first explosion happen. there was some commotion. i saw a fire and smoke. and i didn't know what it was. and then from about me to where that gentleman is standing over there, i literally saw the garbage barrel explode. i saw the flash, the fire, the smoke, and i just ran as fast as i could. >> reporter: cameras were rolling from many angles as the force of the blast actually knocks over at least within of the marathoners. look again. most of the injuries, though, suffered by spectators who came to celebrate the finish. >> a bomb went off. and it knocked me to the ground. and then, you know, everybody started running, panicking. and then another bomb went off. th